This invention relates to an apparatus for detecting a security thread which is embedded in paper-like materials such as paper currencies, checks or securities for the purpose of preventing forgeries.
Copying techniques such as electrostatic copying have recently advanced to such an extent that paper currencies, checks or securities may be easily forged. Countermeasures must be taken, especially against the forgery of paper currencies. For example, a security thread, which may be an elongated nonmetal piece (e.g., a plastic piece) or an elongated metal piece (e.g., an aluminum piece), is embedded in a paper currency. The security thread is effective in preventing forgery since the paper currency can be identified by the presence and quality of the security thread. However, accurate detection of the security thread is hard to achieve. Since a sensor must be kept in close contact with the paper currency to detect the thread by a change in the thickness of the paper money, some creases or folds will inevitably be detected as a security thread. Nor has any apparatus been proposed to distinguish a plastic security thread from an aluminum security thread, and vice versa.